Resolve "Blog posts incorrectly placed in releases blog"

Why is this change being made?

Moves the following blog posts from the release blog to marketing blog and updates categories:

• https://about.gitlab.com/releases/2020/09/01/ci-minutes-update-free-users/ - news
• https://about.gitlab.com/releases/2020/03/18/ci-minutes-for-free-users/ - news
• https://about.gitlab.com/releases/2020/03/12/free-period-for-cicd-external-repositories/ - news
• https://about.gitlab.com/releases/2019/10/03/gitlab-journey-to-cicd/ - engineering
• https://about.gitlab.com/releases/2019/09/11/how-to-avoid-broken-master-with-pipelines-for-merge-requests/ - engineering
• https://about.gitlab.com/releases/2019/07/31/docker-in-docker-with-docker-19-dot-03/ - news
• https://about.gitlab.com/releases/2019/07/16/elasticsearch-update/ - engineering
• https://about.gitlab.com/releases/2018/03/06/reconfigure-inbound-email-for-gitlab-notification/ - news
• https://about.gitlab.com/releases/2017/09/04/old-runners-stop-working/ - news
• https://about.gitlab.com/releases/2020/03/30/gpg-key-for-gitlab-package-repositories-metadata-changing/ - news
• https://about.gitlab.com/releases/2019/07/19/python-3-defailt-for-license-compliance/ - news
• https://about.gitlab.com/releases/2018/06/06/eks-gitlab-integration/ - news
• https://about.gitlab.com/releases/2020/05/06/gitlab-com-13-0-breaking-changes/ - news
• https://about.gitlab.com/releases/2019/04/16/upgrade-runners-for-mr-pipelines/ - news
• https://about.gitlab.com/releases/2020/02/13/upcoming-deprecation-of-secure-analyzer-image/ - news
• https://about.gitlab.com/releases/2019/07/31/docker-in-docker-with-docker-19-dot-03/ - news
• https://about.gitlab.com/releases/2017/09/04/old-runners-stop-working/ - news

Also adds redirects for the release blog post URL to the marketing blog post.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)

Closes #10239 (closed)